(1) Field of the Invention
The present invention relates to Doppler velocity measurements. More particularly, the present invention relates to Doppler current profiler methods and systems that combine measurements from different instances in time from a moving vehicle in order to establish measurements in the same measurement plane.
(2) Description of the Prior Art
It is known that acoustic Doppler current profilers (ADCPs) are used to measure profiles of ocean currents. One common ADCP configuration is the so-called four-beam Janus configuration, having two pairs of opposed transducers. FIG. 1A illustrates a typical prior art ADCP 10 having four transducers 12. The transducers 12 are arranged such that opposing pairs of transducers are centered at 20° or 30° to the axis of the instrument and 40° or 60° to one another.
FIG. 1B is an illustration of a typical prior art deployment of the ADCP 10 from the bottom of a ship or vessel 14, with four acoustic beams 16 from the four transducers 12 projected downward each at an angle of 20° or 30° to vertical, so as to provide two forward directed beams and two aft directed beams.
Each of the acoustic beams 16 measures the Doppler shift of backscattered sound. If the sound-scattering particles move with the water, then the Doppler shift of the backscattered sound from each range cell will be proportional to the radial water velocity or the water velocity component in each range cell that is parallel to the acoustic beam. As used herein and shown in FIG. 1B, a range cell is a discrete volume, or cell, of water at which Doppler measurements are taken. As is known, the distance, or range, to the cell will be a function of the elapsed time from when the sound pulse beam leaves the ADCP to when the backscattered sound is received.
FIG. 1C illustrates the relationship between the acoustic beams 16 and an ocean current 18 for two of the four beams. As is known, the ADCP 10 sends sound pulses 20 from transducers 12 in a regular sequence, receives the backscattered sound 22 in each corresponding transducer, and computes a time series of radial velocity 24 for each beam from the Doppler shifts in a sequence of cells along the length of each beam using range gates.
When installed on a vessel, the beams are normally aligned so that each of the beams is at a small angle (usually φ=20°-30°) to vertical and each is at 450 to the fore and aft axis of the vessel with two beams pointing at ±45° to forward and two pointing ±45° to aft. In the standard processing for the ADCP, radial velocity measurements from the four beams are combined using a trigonometric transformation to compute an orthogonal three-component velocity vector from the four non-orthogonal radial velocities. This procedure is repeated for each of the range cells to compute a time series of the velocity profiles of the two components of horizontal velocity and the vertical velocity.
Some ocean currents are known to be relatively compact or limited in horizontal extent in one direction but also relatively steady and coherent or slowly varying in the other horizontal direction. Such jet-like flows are commonly found in equatorial current systems and western boundary currents such as the Gulf Stream or tidal flows involving topographic features such as flows through straits or over sills. Although measurements from each of the four beams of the Janus ADCP are essentially simultaneous, each of the four beams measures water velocity at a different location in the horizontal plane because of the angular divergence of the acoustic beams. In the conventional configuration, the beam separation increases linearly with distance to the range cell. As a result of this divergence, horizontal variations in the orthogonal velocity components with length scales on the order of the beam separation or less cannot be resolved.